1. Field of the Invention
This invention relates broadly to artificial lens implants for the eye. More particularly, this invention relates to artificial gel-type lens implants which are formed by injection of a prepolymer into the lens capsule for in situ polymerization.
2. State of the Art
One focus of research in cataract surgery is the replacement of the natural crystalline lens of the eye with a gel-type intraocular lens (IOL) that can adjust with contraction and relaxation of the muscles of the eye. The muscles of the eye that control this adjustment are called the zonules. The ideal gel-type replacement intraocular lens is often referred to as a phako-ersatz lens and it differs from the currently used pseudo-phakic lens in that it can theoretically accommodate over 8 diopters of change, whereas the current generation of pseudo-phakic lenses can accommodate at most 2-3 diopters. An example of a commercially-available pseudo-phakic lens is marketed under the trade name Crystalens™, which move slightly forwards or backwards due to pressure built up in the posterior chamber of the eye.
The natural crystalline lens is a gel-like material that sits within the lens capsule of the eye and when the lens capsule is stretched by the zonules, the gel changes its thickness and therefore its focal point thereby allowing focusing at different distances. The phako-ersatz lens must therefore be a gel or viscous liquid (hereinafter referred to as a “gel) with a relatively fast response time. Children have the ability to accommodate over 15 diopters. People over the age of 45 can generally accommodate between 1 to 3 diopters due to the stiffening of the lens which occurs with age. People over 50 generally accommodate less than 2 diopters. This lack of accommodation is called presbyopia.
It is also desirable that the gel be placed in the eye through a small opening. It is more preferable that the gel be injected in the eye through a needle or cannula as a liquid and then converted into a gel by a polymerization reaction (usually initiated by light). Although there are gels that approach these capabilities, such as silicone-based gels, there have been some extraneous limitations that have prevented their success in the field. One of these limitations has been the well-known problem of posterior capsule opacification (PCO).
When the natural lens is removed from the lens capsule, lens epithelial cells (LECs) begin to multiply and spread on the posterior capsule and effectively render the posterior capsule opaque, which results in impaired vision. The LEC's also spread on the anterior wall. However, due to the large opening in the anterior capsule (the capsulorrhexus), there is no wall for them to spread onto. The occurrence of PCO is relatively high in traditional IOL implantations where the LECs spread between the IOL and the lens capsule. There have been IOL designs where the sharpness of the corners of the lens prevents cellular migration under the lens; however, recent literature suggests that these geometrical features simply retard the progression of PCO. PCO occurs in approximately 40% of IOL recipients within two years of receiving a synthetic lens. The usual treatment for PCO is laser ablation of the posterior capsule where a laser is used to vaporize the posterior capsule and the cells that adhere to it. However, when a gel is present in the capsule, as required for a phako-ersatz-type procedure, the posterior wall cannot be ablated as the gelled lens will extrude out of the capsule. Further the phako-ersatz gel does not have sharp corners to prevent LEC migration. Therefore, the presence of PCO has been a major limiting factor in achieving the phako-ersatz lens. Thus, there is a need for a better material to form a phako-ersatz lens that significantly reduces PCO.